1. Field of the Invention
The present invention relates to an electrostatic powder coating method and an electrostatic powder coating apparatus.
2. Description of Related Art
It is known to coat an armature component of a dynamo-electric machine (e.g., an electric motor) with a dielectric resin material to insulate between the armature component and armature coils wound therearound. The coating of the resin material to the armature component is made with, for example, an electrostatic powder coating apparatus (system) recited in, for instance, Japanese Unexamined Patent Publication No. H06-285397A.
At the time of insulating the armature component, the armature component (also referred to as a workpiece) is conveyed to various processes through a screw conveyer in the electrostatic powder coating apparatus. First of all, the armature component is conveyed to a heating and degreasing process. In the heating and degreasing process, the armature component is heated by a high-frequency induction heating device to heat and degrease the armature component. Next, the armature component is conveyed to a cooling process, at which the armature component is cooled with the air blown thereto. Thereafter, the armature component is conveyed to a coating process, at which dielectric resin powder, such as epoxy resin powder, is applied from a coating device to the armature component. In this way, the surface of the armature component is coated with the resin powder. Next, the armature component is conveyed to a heating and curing process, at which the armature component is heated by a high-frequency swing heating device. In this way, the resin powder, which adheres to the surface of the armature component, is heat cured and is thereby fixed to the surface of the armature component. Finally, the armature component is conveyed to another cooling process, at which the armature component is cooled with the air blown thereto.
FIG. 8A shows a result of an experiment, in which armature components are conveyed continuously one after another at equal intervals by the screw conveyer and are heated by the high-frequency induction heating device. More specifically, FIG. 8A shows a relationship between a temperature difference between a first one and a center one of the armature components in a row of the armature components on the screw conveyer in the high-frequency induction heating device and an interval of the armature components in the row of the armature components. With reference to FIG. 8B, an interval D2 of the armature components 101 is a distance between a central axis (center) of one of two adjacent armature components 101 and a central axis (center) of the other one of the two adjacent armature components 101. The interval D2 is set to be a multiple of a width W3 of the armature component 101 in the conveying direction. With reference to FIG. 8A, it should be noted that the temperature difference between the first one and the center one of the armature components is smaller as the interval D2 between the armature components becomes longer. Therefore, when the interval between the armature components is reduced to reduce the entire size of the electrostatic powder coating apparatus, the variation in the temperature of the armature components conveyed by the screw conveyer becomes large in the heating and degreasing process and the heating and curing process. Thus, a degree (state) of degreasing of the armature component and a degree (state) of curing of the resin powder may possibly vary from product to product, thereby possibly resulting in variations in the quality of the armature components after the powder coating.
Furthermore, in order to reduce the size of the electrostatic powder coating apparatus, it is desirable to supply the alternating current from a single high frequency oscillator to the heating coil used in the heating and degreasing process and also to the heating coil used in the heating and curing process. When the alternating current is simultaneously supplied from the single high-frequency oscillator to the two heating coils, there is substantially no temperature difference between the armature components in the heating and degreasing process and the armature components in the heating and curing process as long as the screw conveyer is fully loaded with the armature components in these two processes. However, in a case where the number of the armature components conveyed by the screw conveyer in the heating and degreasing process is reduced and thereby becomes smaller while the armature components are kept fully loaded on the screw conveyer in the heating and curing process, the temperature of the armature components conveyed by the screw conveyer in the heating and degreasing process become higher than the temperature of the armature components conveyed by the screw conveyer in the heating and curing process. Similarly, in a case where the armature components are kept fully loaded on the screw conveyer in the heating and degreasing process while the number of the armature components conveyed by the screw conveyer in the heating and curing process is reduced and thereby becomes smaller, the temperature of the armature components conveyed by the screw conveyer in the heating and curing process becomes higher than the temperature of the armature components conveyed by the screw conveyer in the heating and degreasing process. Furthermore, in the heating and degreasing process and the heating and curing process, when the number of components on the screw conveyer is reduced in both of the heating and degreasing process and the heating and curing process, the temperature of the armature components is raised in both of the heating and degreasing process and the heating and curing process. When the temperature of the armature component in the heating and degreasing process varies among the armature components, the degree (state) of degreasing of the armature component varies among the armature components. Furthermore, when the temperature of the armature component in the heating and curing process varies among the armature components, the degree (state) of curing of the armature component also varies among the armature components. Therefore, the quality of the armature component may vary among the armature components. Furthermore, when the temperature in the heating and degreasing process and the temperature in the heating and curing process become excessively high, the heat resistance property of the screw conveyer may possibly be deteriorated.
Furthermore, in Japanese Unexamined Patent Publication No. H06-285397A, the electrostatic powder coating apparatus includes an electrode receiver, a storage container and a vacuum box. The electrode receiver receives an electrode. The storage container is placed above the electrode receiver and stores resin powder. The vacuum box is placed above the storage container. The armature component is conveyed into the vacuum box, and the resin powder is upwardly sprayed along the compressed air against the armature component from the lower side of the armature component to coat the resin powder on the surface of the armature component. At this time, a negative voltage is applied to the electrode, so that the resin powder is placed in the negatively charged state. In contrast, the armature component is positively charged. Therefore, the resin powder electrostatically adheres to the surface of the armature component. The resin powder, which did not adhere to the armature component, is drawn out of the vacuum box from an upper part of the vacuum box and is recovered.
Japanese Unexamined Patent Publication No. 2005-138048A (corresponding to U.S. Pat. No. 7,371,284B2) teaches another type of electrostatic powder coating apparatus. In this electrostatic powder coating apparatus, the resin powder, which is positively charged, is downwardly sprayed onto an armature component, which is negatively charged, from the upper side of the armature component, so that the resin powder is coated on the surface of the armature component.
FIG. 10 is a diagram indicating a histogram of a particle size of the new resin powder (virgin material) and a histogram of a particle size of the collected resin powder, which is collected from the storage container after upwardly spraying the resin powder to the armature component 1 from the lower side of the armature component 1 in the electrostatic powder coating apparatus of Japanese Unexamined Patent Publication No. H06-285397A. As is understood from FIG. 10, in the case of the electrostatic powder coating apparatus, which upwardly sprays the resin powder to the armature component from the lower side of the armature component, the small powder particles having small particle sizes tend to be increased in the storage container in comparison to the large powder particles having large particle sizes. This is due to the following reason. That is, since the small powder particles cannot be easily adhered to the surface of the armature component, the small powder particles fell down without adhering to the surface of the armature component. When the quantity of the small powder particles is increased in the storage container, the stable adhesion of the resin powder to the surface of the armature component becomes difficult. Therefore, the film thickness of the dielectric insulation film, which is formed on the surface of the armature component, may vary from product to product. Furthermore, the small powder particles of the resin powder tend to cause an insufficiency of the film thickness of the dielectric insulation film.
Furthermore, it is lately demanded to recycle the resin powder, which was sprayed to the armature component and did not adhere to the armature component, for the next spraying of the resin powder. In the electrostatic powder coating apparatus of Japanese Unexamined Patent Publication No. 2005-138048A (corresponding to U.S. Pat. No. 7,371,284B2), the resin powder is downwardly sprayed onto the armature component from the upper side of the armature component. However, it does not teach a specific way of recycling the resin powder.